Process of making metallic molding powders



y 4:, 1940. L. G. KLINKER 2,200,369

PROCESS OF MAKING METALLIC MOLDING POWDERS Original Filed July 18, 1958INVENTOR JLAM MW F Patented May 14, 1940 PROCESS OF MAKING METALLICMOLDIN POWDERS Louis G. Klinker, New Gastle, Pa., assignor to JohnsonBronze Company, New Castle, Pa., a corporation of Pennsylvania.

Application July 18, 1938, Serial No. 219,937 Renewed December 11, 19393 Claims.

The present invention relates to metallic molding powders and thepreparation thereof, and more especially for powders which are coldmolded into metallic articles which are sintered after molding. Variousarticles are now made by the powder molding process in which a metallicpowder is compressed to form the article and the article is then heatedto a temperature sufiicient to cause a sintering of the metallicparticles. In the manufacture of molded articles from metal powders ithas been known that electrolytic metal powders cold mold into greenpieces having more strength for a given mold pressure than powdersformed by grinding metals or reducing them chemically. Thischaracteristic of electrolytically formed powders is believed to be dueto their dendritic structure which gives rough surfaces and interlockwhen the molding pressure is applied.

One purpose of my process is the preparation of a powder which has therough surface or dendritic-like characteristics of electrolyticallyformed metal powders but produced from a ground or otherwise naturallysmooth surface powder. In general, I prefer to form the basic metalpowder by a grinding operation. With most metals the powder producedconsists of essentially smooth and more or less spherical particles.This powder is ground to a fine size, usually so that all of theparticles will pass a 100 or 150 mesh to the inch screen. The grindingoperation is preferably carried out in an oxidizing atmosphere, whichresults in a fairly good coating of metal oxide on the particles. Thispowder is then mixed with another and preferably considerably finermetal powder, and the mixture is heated in a reducing atmosphere so asto cause a sintering of the finer particles of the second metallicpowder upon the smooth surfaces of the particles of the first powder.The particles of the second metal powder are sintered to the surfacessomewhat like tentacles, so that when the particles are cold molded,these tentacle-like projections interlock to give a greater greenstrength. I have also found that this greater strength persists in thefinally formed articles.

The sintering to roughen the surfaces of the particles and give them themolding characteristics of the dendritic electrolytic powders is carriedout at a temperature and sufficient length of time to cause theparticles to become thoroughly sintered together. This results in asintered mass which, after cooling, is reground to serve as a moldingpowder for making the finished articles.

In the drawing- Fig. 1 is a photomicrograph of particles of metalproduced by the grinding process; Fig. 2 is a photomicrograph of finermetal particles used for sintering to the surfaces of the particlesshown in Fig. 1;

Fig. 3 is a photomicrograph of particles of the type shown in Fig. 1with particles of the type shown in Fig. 2 sintered to their surfaces.

I will now explain the present invention further with particularreference to certain specific embodiments.

Small articles of irregular outline such, for example as latch platesfor automobile bodies, are made by the powder process from powderedsteel or iron and binding metal such as copper, since by this processthey can be made cheaper than by casting and machining and havesuflicient strength for the purposes required. As a specific example, Iprepare a powder by grinding steel or cast iron to a fineness to passthrough about a 100 mesh screen. The grinding is carried out in the airand the particles become coated with iron oxide. A second iron powder,preferably of a somewhat dissimilar composition and of a finer mesh, saythrough about a 250 mesh screen, is prepared and mixed with the firstpowder. The powder is then placed in a furnace and heated in a reducingatmosphere to a temperature sufiicient to cause the iron particles tobecome sintered together. During the sintering treatment in a reducingatmosphere, the iron oxide on the surface of the particles becomesreduced to metallic iron and I have found that this 'is of considerableadvantage in causing a rapid and firm sintering of the smaller particlesto the surfaces of the larger particles so as to give them a rougheneddendritic or tentacle-like surface. After the sintering is completed,the mass is cooled and ground to form the molding powder, which consistslargely of the ground particles which were initially smooth but thesurfaces of which now are roughened by the sintering thereto of thesmaller particles of metal. This powder has, in general, the moldingcharacteristics of powders produced electrolytically and may besubstituted therefor in the usual molding and sintering operations 50 toform the finished articles according to well known practice.

Another specific example is the preparation of powders used for makingbronze bearings by the powder molding process. Bronze, preferably g5waste turnings or borlngs from the making of cast bronze bearings, isground to a fine powder, preferably to. all pass a 100 or 150 meshscreen. The grinding is carried out under oxidizing conditions so thatthe particles of this powder are coated with a film of oxide. A finelypowdered copper is prepared, preferably of a somewhat smaller mesh size,and mixed with the bronze powder. The mixture is then heated in areducing atmosphere to a temperature sumcient to cause the copperparticles to become sintercd to the bronze particles and copperize them.This temperature is between 500 and 1800 F. and

preferably in the neighborhood of 1300 F. It should be below the fusingpoint of the bronze or the copper but high enough to cause sintering ofthe copper particles to the surface of the bronze particles. The oxidefilm on the bronze particles is reduced during the sintering treatmentand the nascent reduced metal apparently contributes to a ready and veryadherent sintering of the copper particles to the surface of the bronzeparticles. I

The sintered mass is ground to form a molding powder to be used formaking bronze bearings by any suitable powder molding processI In thedrawing, Fig. 1 is a photomicrograph of particles of the bronze powderproduced by grinding, and it will be noted that these particles are of ageneral nodular or spherical shape and have smooth surfaces. Fig. 2 is aphotomicrograph of the powdered copper, which is preferably ground tofiner grain size. Fig. 3 is a photomicrograph of the particles of theoopperized molding powder after the initial sintering and afterregrinding. It will be noted that these particles have roughenedsurfaces formed by the adhesion and sintering to the initially smoothsurfaces of the bronze particles of the smaller copper particles.

In the manufacture of bearings from the copperized bronze powder, thebronze powder is preferably mixed with a small amount, say about 5% ofpowdered tin, and the articles cold molded and sintered into bearings. I

While it is preferred to use a dissimilar metal as the finer coatingpowder, a single metal may be ground to form a mixture of coarser andfiner particles and the mixtures sintered so as to cause the finerparticles to be sintercd to and roughen the surfaces of the largerparticles.

While it is preferred to mix the basic metal powder with a second andmore finely ground metal powder and sinter the particles of the secondmetal to the surfaces of the particles of the first metal to roughenthem, a roughening metal may be otherwise deposited and sintered, as bymixing the first powder with-a reducible compound of the second metal.For example, in place i of iron, iron-oxide may be used, or in place ofof the article.

ble metal compounds may be employed instead of the'oxides, such as themetallic compounds, and in the case of copper, copper cyanide, copperoxalate, etc., may be used. Under a. reducing atmosphere these metalliccompounds are reduced to yield minute metal particles, which aresintered to the surfaces of the basic metal particles.

While I prefer to mechanically grind or disintegrate the metal to formthe powders, the powders may be formed otherwise, as by reduction ofmetal compounds by heat, such as by heating iron carbonyl whichdecomposes forming spherical iron powder particles. Such processes, ingeneral, yield powders having smoother surfaces than electrolyticallyformed powders and such powders are greatly improved by our treat mentfor molding purposes. Such powders are heated in an oxidizing atmosphereto form oxide films on the particles before they are used in anyprocess.

The green articles produced (the articles cold molded before sintering)have a greater strength than those formed of powders produced bygrinding. Also the finished sintercd articles have a greater strength,since the interlocking of the rough surfaces apparently not onlycontributes to the green strength but also to the final strength Inmaking the finished articles, my powders may be used in usual moldingoperations either by themselves or being mixed with other powders.

'While I have specifically described the preferred embodiments of myinvention, it is to be understood that the invention may be otherwiseembodied and practiced within the scope of the following claims.

I claim: l. The process of making metallic molding powders whichcomprises grinding a metal under oxidizing conditions to form anoxide-coatedl powder, mixing with it a second powder of a material ofthe class consisting of metals and reducible metal compounds, sinteringthe mixture in a reducing atmosphere, and regrinding.

2. The process of making metallic molding powders which comprisesforming a powdered metal having smooth oxide-coated particles, mixing itwith another powdered metal and heating the mixture in a reducingatmosphere to reduce the oxide coating and sinter the metallic particlesand roughen the smooth surfaces, and reducing the sintercd mass to amolding powder.

3. The process of making metallic molding powders which comprisesforming a powdered metal having oxide-coated particles, mixing with .58

it a material of the class consisting'of finely powdered metals andreducible metal compounds, sintering the mixture in a reducingatmosphere, and pulverizing.

LOUIS G. R.

